1. Field of the Invention
The present invention relates to a development apparatus used for forming an image using an electrophotographic system or an electrostatic recording system, and more particularly, to a development apparatus having a plurality of developer bearing members.
2. Description of the Related Art
In an image forming apparatus, such as an electrophotographic copying machine, a magnetic brush developing method using a developing sleeve which is a developer bearing member is generally used. The magnetic brush developing method will be described below.
First, to efficiently develop an electrostatic image on a photosensitive drum, binary-component developer or one-component developer is held by a developing sleeve. The developing sleeve is a hollow cylindrical developer bearing member made of non-magnetic body having a magnetic pole therein.
Next, developer is conveyed from a developer container to a developing region opposed to the photosensitive drum. Developer is an ear of magnetic developer in the developing region by effect of a magnetic field, the developer slides on a photosensitive drum surface, thereby developing an electrostatic latent image formed on the photosensitive drum.
The magnetic brush developing method using the developing sleeve is used in many products, such as a monochrome digital copying machine and a full-color copying machine requiring high image quality.
When rotation moving speed of the photosensitive drum is relatively low, i.e., in the case of a relatively low speed copying machine, since an excellent developed image is obtained even if developing time is short, one developing sleeve is sufficient. However, when the rotation moving speed of the photosensitive drum is increased due to high speed demand of recent copying machine, an excellent image is not always formed using one developing sleeve.
As a countermeasure for this, there is a method for enhancing the developing efficiency by increasing the circumferential velocity of the developing sleeve. However, if the circumferential velocity of the developing sleeve is increased, a centrifugal force acting on the developer which forms a magnetic brush is increased, scatter of developer is increased, the inside of the copying machine is contaminated, and there is an adverse possibility that the function of the apparatus is deteriorated.
As another countermeasure, there is proposed a so-called multi-magnetic brush developing method in which two or more developing sleeves are used, they are disposed such that peripheral surfaces thereof are close and adjacent to each other, developer is conveyed such that the developer runs over the peripheral surfaces, the developing time is increased, and developing ability is enhanced (see Japanese Patent No. 02699968).
In the multi-magnetic brush developing type development apparatus having two or more developing sleeves, there is a well known development apparatus using either stainless steel (SUS) or aluminum as a material of base material of the developing sleeve.
However, the development apparatus using a stainless steel developing sleeve has the following problems.
Stainless steel has relatively high hardness and thus has excellent durability, but has a drawback in that thermal conductivity is poor. Therefore, if there is a heat source in the vicinity of the developing sleeve, a temperature difference is generated between the heat source side of the developing sleeve and the opposite side, and the developing sleeve is deformed by heat, so that developing density is varied in the rotation cycle of the developing sleeve in some cases.
As a countermeasure of this problem, there is proposed a method in which the developing sleeve is rotated little by little at certain time intervals during standby of the image forming apparatus, a copying operation is started after variation in the temperature distribution of the developing sleeve is reduced. However, this method has a problem that lifetime of a driving system of the developing sleeve is shortened and that developer is scattered.
There is a proposed method in which at the same time when a user pushes a copy button, the developing sleeve is rotated only during a certain period to eliminate the variation in temperature distribution of the developing sleeve, and then the copying operation is started. However, this method has a problem that the first copy time is increased. Therefore, it is necessary to make uniform the temperature distribution of the developing sleeve as much as possible.
The development apparatus using an aluminum developing sleeve has the following problem.
Contrary to stainless steel, aluminum has high thermal conductivity and is relatively soft. Therefore, satisfactory durability cannot be obtained in some cases.